Eibar drew level just before halftime as Cuevas headed past Casillas from Oscar Perez's corner.
The Basque team came close to winning the match when midfielder Francisco Jose Corredoira rattled the bar in the 80th minute with a long-range shot.
Osasuna, which reached the semifinals last season, maintained its hopes of repeating the feat by drawing on its visit to Valencia.
Forward Miguel Angel Angulo gave Valencia the lead in the 27th minute when he shot home after a mix-up in the Osasuna defense.
Valencia played the second half with 10 men when right back Curro Torres received a yellow card for a foul on Cameroon forward Pierre Webo just before the interval and was then ejected, apparently for protesting. Despite the numerical disadvantage, Spanish international midfielder Ruben Baraja doubled Valencia's lead with a 54th-minute penalty.
Osasuna reduced the deficit six minutes later when Webo, looking offside, turned a shot by Argentine midfielder Gerardo Rivero over the line.
Aloisi, who had replaced midfielder Cesar Palacios eight minutes earlier, pounced just before time to earn Osasuna a deserved draw.
Serbian and Montenegrin striker Goran Drulic scored twice as Zaragoza gained a convincing win over its fellow league struggler Betis.
Drulic opened the scoring in the 50th minute, although Betis' Spanish international winger Joaquin Sanchez leveled with a header six minutes later.
Zaragoza regained the lead through a fierce strike 10 minutes from time by midfielder Martin Vellisca, with Drulic striking again just before time.
Celta de Vigo, languishing in the relegation zone, relieved the pressure on its coach Miguel Angel Lotina with a narrow win in Malaga.
Brazilian midfielder Vagner scored the game's only goal with a lofted long-range shot in the 55th minute.
Second-division Alaves kept its hopes alive of progressing to the last eight of the competition with a goalless home draw with Valladolid.
Alaves survived the loss of its defender Juan Cruz Ochoa, who was dismissed midway through the second half.
On Tuesday, Atletico de Madrid drew 0-0 with Deportivo de La Coruna.
On Thursday, second-division leader Levante entertains off-form FC Barcelona and Villarreal plays Sevilla.
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